1. Field of the Invention
The present invention relates to a mold for a golf ball and a method of manufacturing a golf ball. More particularly, the present invention relates to an improvement in a mold to be used for forming the cover of a golf ball.
2. Description of the Related Art
A two pieces golf ball is constituted by a core and a cover covering the surface of the core. In the manufacture of the two pieces golf ball, a mold including an upper mold half and a lower mold half which have hemispherical cavities is used. By uniting the upper mold half and the lower mold half, a spherical cavity is formed. The mold includes a support pin capable of being moved forward and backward with respect to the cavity and a vent pin for causing the cavity and outside air to communicate with each other. The vent pins are provided on the poles of the upper mold half and the lower mold half. By the support pins which are moved forward, the core is held on the center of the cavity. Then, a resin composition is put in an injection molding machine and is thus molten, and is injected toward the cavity through a plurality of gates. The resin composition thus molten is filled in a portion between a cavity surface and the core. The molten resin composition covers the core and coagulates to form a cover shortly.
In a stage in which the core is held by the support pin, air is present between the cavity surface and the core. When the molten resin composition is injected into the cavity, the air in the cavity is discharged to an outside. The discharge is carried out through a clearance of the support pin and the vent pin. There has also been proposed a mold in which a support pin formed by a porous material is used and the air is discharged from the support pin (Japanese Laid-Open Patent Publication No. 2000-37480).
If the discharge of the air is insufficient, there are caused defects such as the generation of bare, the intrusion of the air into the cover, the scorching of a resin or the generation of a weld mark. In the formation of a thin cover, the molten resin composition is fluidized with difficulty. Consequently, the defects are particularly apt to be caused.
The tip of the support pin which is moved forward is protruded from the cavity surface. The tip hinders the fluidization of the molten resin composition. If the fluidization is insufficient, the defects are apt to be caused.
In some cases in which the support pin is moved backward, the molten resin composition stuck onto the surface of the support pin is brought into the clearance. When the molten resin composition remains, the discharge of the air from the clearance is inhibited. The invasion of the molten resin composition into the clearance highly increases the proportion defective of a golf ball.
In recent years, a golf ball including a cover having a small thickness has been manufactured. In the formation of the cover, a molten resin composition tends to be non-fluidized smoothly due to a small interval between the cavity surface and the core. In the formation of the cover having a small thickness, generally, a polymer having a low viscosity is selected or the temperature of a resin is set to be high in order to enhance the fluidity of the molten resin composition. The molten resin composition having an excellent fluidity is apt to enter the clearance. In the manufacture of the golf ball including the cover having a small thickness, a yield is low.
There is also a golf ball including a cover formed by at least two layers. Also in the formation of each layer of the golf ball, there is a problem in that the air remains.
It is an object of the present invention to provide a mold for a golf ball in which air is easily discharged. It is another object of the present invention to provide a method of manufacturing a golf ball in which a golf ball of good quality can be obtained and a yield is high.